3D labels weren't selectable.
Shape selection working.
Wasn't deleting layers properly.
WhirlyGlobeLib/src/LayerThread.mm

Fixed a shutdown race condition.
Paging layers were going insane around the edges of the 2D map.  Was a logic error in the MaplyView updates.
Restricting options that don't work for flat maps.  Fixed the config popcontrol.
Exposed max tiles in quad image tiles layer.
Fixed a layer shutdown problem in non-main layer threads.
First pass at the offline renderer.
Merge branch 'develop' of github.com:mousebird/WhirlyGlobe into develop